System for adsorption measurement

ABSTRACT

The system measures by the adsorption of gas the specific surfaces of porous or divided solids. It injects equal quantities of adsorbate in stages into a sample-carrying chamber. These equal quantities of adsorbate come from a chamber of constant volume in which, before injection into the sample-carrying chamber, the adsorbate pressure is in each case equal to the pressure of equilibrium of the adsorption of the previous injection increased by a constant value.

United States Patent {191 Glaude et a1.

[ May 15, 1973 I54] SYSTEM FOR ADSORPTION MEASUREMENT [75] Inventors:Philemon Glaude, Groet; Walter Karcher, Alkmaar; Robert Smets, Groet,all of Netherlands [73] Assignee: European Atomic Energy Community(Euratom), Luxembourg [22] Filed: Dec. 4, 1970 [21] Appl. No: 95,316

[30] Foreign Application Priority Data Dec. 10, 1969 Luxembourg ..59979[52] US. Cl ..73/432 PS [51] Int. Cl. ..G0ln 15/08 [58] Field of Search..73/432 PS, 38, 61 R [56] References Cited UNITED STATES PATENTS3,555,912 1/1971 Lowell ..73/432 PS Orr, Jr. et al ..73/432 PSCoggeshall et a]. ..73/432 PS FOREIGN PATENTS OR APPLICATIONS 1,202,54010/1965 Germany ..73/432PS Primary ExaminerLouis R, Prince AssistantExaminer-Joseph W. Roskos Attorney-Stevens. Davis, Miller & Mosher [57]ABSTRACT The system measures by the adsorption of gas the specificsurfaces of porous or divided solids. It injects equal quantities ofadsorbate in stages into a samplecarrying chamber. These equalquantities of adsorbate come from a chamber of constant volume in which,before injection into the sample-carrying chamber, the adsorbatepressure is in each case equal to the pressure of equilibrium of theadsorption of the previous injection increased by a constant value.

7 Claims, 1 Drawing Figure SYSTEM FOR ABSORPTION MEASUREMENT Thisinvention relates to a system for adsorption measurement.

When it is desired to characterize the texture of porous or dividedsolids, very valuable information is provided by measurements ofspecific surfaces and distributions of pore radii. Methods used includemeasurement of adsorption of gas at low temperature. This providesinformation on the distribution of the sizes of pores having radiibetween 20 and 350 A, the pore range to which BET surfaces are due. Italso yields reproducible measurement results. That is why this method ofmeasurement is the most widely used.

Various types of automatic systems for the measurement of adsorption areavailable on the market. They are one or other of the methods describedbelow:

a. The dynamic volumetric method of adsorption. This enables smallsamples to be used, but has the disadvantage of recording nonstabilizedequilibrium pressures, especially in the case of microporous substances.

b. The gravimetric method, which also enables small samples to be used.This comprises measuring the vari ation of the weight of the sampleaccording to the pressure of the ambient adsorbate. The method has thedisadvantage of measuring adsorption values that are systematicallylower than those measured by volumetric methods. This difference is dueto the difficulty of keeping the sample at the adsorption temperature.

c. The dynamic method of differential adsorption. The adsorbate isintroduced continuously at known rates into two sample-carriers, one ofwhich contains a reference substance (a non-porous substance of knownsurface). This method necessitates the use of large surfaces and has thedisadvantage already mentioned for a dynamic method, i.e. of recordingnon-stabilized equilibrium pressures.

d. The static volumetric method. This is used in the system according tothe present invention. It consists of injecting successive and equalquantities of adsorbate into the sample-carrying chamber. After eachinjection of adsorbate, the equilibrium pressure is allowed to establishitself and is then measured. Knowledge of the different successiveequilibrium pressures during a complete cycle of adsorptions anddesorptions enables the specific surface of the sample to be determinedtogether with the distribution of pores having radii between 20 and 350A by a calculation that can readily be carried out on a small officeelectronic computer.

According to the known method used hitherto, equal quantities ofadsorbate are introduced into the samplecarrying chamber as follows.Adsorbate is injected into a cylindrical chamber at a constant pressure.When the chamber is filled with adsorbate, a plunger drives the latterfrom the chamber to the sample-carrying chamber. The disadvantage ofthis system is immediately apparent. It is impossible to ensure that thequantity of adsorbate injected into the sample-carrying chamber isexactly the same from one injection to the next. This is because thesystem is mechanical, the travel of the plunger varies a little on eachoccasion whatever precautions are taken, and impurities, for exampleair, can enter the adsorbate. Consequently, to minimize this lack ofprecision or the introduction ofimpurities, measurements can only betaken with large samples.

According to the present invention there is provided a system formeasuring the adsorption of a gaseous adsorbate by an adsorbingsubstance using the static volumetric method of adsorption by successivestages, which system comprises the introduction of equal quantities ofadsorbate into a sample-carrying chamber in successive stages, whereinsaid equal quantities of adsorbate are introduced to the sample-carryingchamber from a chamber of constant volume which is separated from thesample-carrying chamber by a valve, said valve being open during eachstage to enable the pressures in the two chambers to be equilibrated andclosed before the following stage to permit a rise of a predeterminedconstant value in the pressure of adsorbate in the constant volumechamber.

The system according to the present invention, as.

specifically exemplified hereafter, enables small samples to be used,weighing less than one gram. It enables very small and very exactlycontrolled and equal quantities of adsorbate of the order of 0.1 cm NTPto be introduced into the sample-carrying chamber. The system accordingto the present invention is noteworthy in that these equal quantities ofadsorbate come from a chamber of constant volume separated from thesample-carrying chamber by a valve, which is open during each stage andenables the pressures in the two chambers to be equilibrated and, closedbefore the next stage, permits a rise in the pressure of a predeterminedconstant value Ap in the chamber of constant volume.

The invention will be understood better by means of the following textand with reference to the single FIG- URE, which is a diagram by way ofexample of a specific system in accordance with the invention.

The FIGURE shows the sample-carrying chamber 1. This is cooled to theadsorption temperature by an appropriate means (not shown), which may bea simple Dewar device containing a cooling liquid whose level is keptconstant. The chamber 1 is connected by capillaries 2,3 and 4 to achamber 5 of constant volume and to the reference part of a manostat 6fitted as a differential pressure regulator. The chamber of constantvolume 5 is bounded by three valves 7,8 and 9 and by the diaphragm of apressure pick-up 10, which is connected to a vacuum pump 11 (not shown).The latter is used for giving a constant pressure reference. Atmosphericpressure, which is variable, cannot be used as a pressure referenceduring measurement that may take 24 hours.

The valve 9 connects the chamber 5 to a calibrated space 12 into whichthere may be introduced, by means of a valve 13, a gas such as helium,which is brought from a source 14 (not shown). Determination of thevolume of the chambers 5 and 1 together with the capillaries thatconnect them and the interior of the manostat 6 is effected by expansionof the helium in the calibrated space 12- to the chambers andcapillaries. This volume is readily determined by means of the law pVconstant.

.The valve 7 communicates with a three-way valve 15, which is connectedto a vacuum pump 16 (not shown) and to the regulating part of themanostat via a duct 17. The adsorbate coming from a source 19 (notshown) is introduced into this part of the manostat through a valve 18.A sintered-glass plate 24 enables excess gas to escape into the pump 22(not shown) without permitting the passage of the mercury 25. A mercuryreservoir (not shown) communicates with a valve 23, enabling theselected Ap to be adjusted or the mercury to be removed from themanostat on the degassing of the sample or when the system is no longerin use. The pressure difference Ap is regulated by the height of themercury column above its free level in the manostat 6.

The walls of the chamber 12 are advantageously made of transparentmaterial. This makes it easy to check that no air bubbles are includedin the liquid (which would falsify measurements) when the chamber isfilled with liquid (which is weighed afterwards), in order to determineits volume with great precision.

The valves 7 and 8 are controlled by two clocks, which control eachother in closed circuit, i.e. one is started when the other stops. Thusthese two valves are opened and closed automatically without the needfor the presence of a human observer.

The system operates as follows. The adsorbate flows continuously intothe space bounded by the valves 18, 20 and 7, in which a pressure higherby Ap than the one in the chamber 1 is kept constant by means of themanostat. A vacuum having previously been produced in the chambers l and5, the pressure of adsorbate is therefore equal to Ap. When the valve 7is opened the chamber is filled with adsorbate at that pressure Ap. Whenthe valve 7 is closed and the valve 8 is opened the adsorbate in thechamber of constant volume flows towards the capillaries 3,4 and 2 andto the samplecarrying chamber 1, in which it is partly adsorbed by thesample. The quantity of adsorbate introduced into theadsorption-measuring part (chamber 5, capillaries 3,4 and 2 andchamber 1) during this first adsorption stage corresponds to the volumeof the chamber of constant volume 5 at the pressure Ap. The valve 8remains open during the time necessary for adsorption to be stabilized.The pressure in the chamber 1 then becomes p,.

The pressure p acting on the reference part of the manostat adjusts toAp p the pressure of the adsorbate in the space bounded by the valves18, and 7. After the conclusion of the first adsorption stage the valve8 is also closed. The valve 7 is then opened, and the chamber 5 isfilled with adsorbate at the pressure Ap p The fresh quantity ofadsorbate brought into the chamber 5 is therefore again that whichcorresponds to the volume (constant) of the chamber 5 at the pressureAp. The valve 7 is then closed, and the valve 8 is opened. The samequantity of adsorbate then flows to the chamber 1, in which a newequilibrium pressure p is established after a certain time of adsorptionin the sample.

In the third stage adsorbate at the pressure p Ap is introduced into thechamber 5 and from there to the chamber 1. The quantities of adsorbateintroduced into the system are therefore the same at each stage. Thesuccession of adsorption stages is controlled solely by the openingofclosing of the valves 7 and 8. When their sequences of opening andclosing are controlled by clocks as in a preferred embodiment of theinvention, the presence of a human observer is no longer necessary.

The (predetermined) number of stages is controlled by any appropriatemeans, for example an electric counter. When this number is reached, thevalve 15 (also controlled automatically) brings the space between thevalves 7 and 15 into constant communication with the vacuum pump 16.Desorption then begins. The opening and closing cycle of the valves 7and 8 begins again unchanged, and the succession of desorption stagesproceeds by successive filling and emptying of desorbed gas.

During all these adsorption and desorption stages, the pressure in thechamber 5 or in the chambers 5 and 1 (together with the capillaries)brought into communication by opening the valve 8 is measured by thepressure pick-up 10. The temperature of the system is kept constant withan accuracy ofi- O.1C.

The advantages of this system will be apparent from the abovedescription. By using the chamber 5, the introduction of very smallquantities of gas, which are exactly known and are equal, without theintroduction of impurities, is possible in the chamber 1. This isbecause, apart from the valves 7 and 8, no moving parts are used in thesystem. Consequently, very small samples can be used, and accuratemeasurements can be taken on absolute surfaces ranging from 0.5 m toseveral thousand square meters BET. Many types of adsorbate, inparticular argon and nitrogen, may be used.

The disposition of the pressure pick-up in the chamber 5 enables theincreases in pressure Ap in that space to be recorded exactly and theequilibrium pressures of adsorption to be recorded as well. Thus, asingle detector calibration curve is necessary, and the operator alwayshas all the data he needs, hence complete recording of all the pressurevariations of the adsorbate.

The signal from the pick-up may also be transmitted to a computer, whichautomatically makes calculations of the BET surface and the distributionof the pore sizes according to a predetermined program.

What we claim is:

1. Apparatus for measuring the adsorption of a gaseous adsorbate by anadsorbing sample using the static volumetric method of adsorption bysuccessive stages, said apparatus comprising a chamber of constantvolume, a sample carrying chamber a first valve connecting the twochambers, an adsorbate pressure controlling means, a second valveconnecting the chamber of constant volume to the pressure controllingmeans, pressure pick-up means connected to said chamber of constantvolume, and means for connecting a source of adsorbate to said pressurecontrolling means, the chamber of constant volume serving for theintroduction of equal quantities of adsorbate into the sample carryingchamber when said first valve is open whereby the pressure of the twochambers are equilibrates, and after closing said first valve thepressure controlling means permitting the pressure of adsorbate in thechamber of constant volume to rise with a predetermined constant valueif said second valve is open, the pressure pick-up means recording thepressure in the chamber of constant volume before and after an adsorbingoperation.

2. Apparatus as claimed in claim 1, wherein the adsorbate pressurecontrolling means is a manostat operating as a differential pressureregulator, the reference part of which is connected to the samplecarrying chamber whereas the regulating part permits the rise ofpressure in the chamber of constant volume with said predeterminedvalue.

3. Apparatus as claimed in claim 2, wherein the regulating part of themanostat communicates by means of a valve with the source of adsorbateand also with a T duct communicating, by means of a valve, with a vacuumpump and by means of a three-way valve, either with the vacuum pump orwith the second valve bounding the entry to the chamber of constantvolume.

6. Apparatus as claimed in claim 5, including two clocks, interconnectedfor controlling the opening and closing of said first and said secondvalve.

7. Apparatus as claimed in claim 6, wherein the calibrated space, themanostat and the sample carrying chamber are made of transparentmaterial.

1. Apparatus for measuring the adsorption of a gaseous adsorbate by anadsorbing sample using the static volumetric method of adsorption bysuccessive stages, said apparatus comprising a chamber of constantvolume, a sample carrying chamber a first valve connecting the twochambers, an adsorbate pressure controlling means, a second valveconnecting the chamber of constant volume to the pressure controllingmeans, pressure pickup means connected to said chamber of constantvolume, and means for connecting a source of adsorbate to said pressurecontrolling means, the chamber of constant volume serving for theintroduction of equal quantities of adsorbate into the sample carryingchamber when said first valve is open whereby the pressure of the twochambers are equilibrates, and after closing said first valve thepressure controlling means permitting the pressure of adsorbate in thechamber of constant volume to rise with a predetermined constant valueif said second valve is open, the pressure pick-up means recording thepressure in the chamber of constant volume before and after an adsorbingoperation.
 2. Apparatus as claimed in claim 1, wherein the adsorbatepressure controlling means is a manostat operating as a differentialpressure regulator, the reference part of which is connected to thesample carrying chamber whereas the regulating part permits the rise ofpressure in the chamber of constant volume with said predeterminedvalue.
 3. Apparatus as claimed in claim 2, wherein the regulating partof the manostat communicates by means of a valve with the source ofadsorbate and also with a T duct communicating, by means of a valve,with a vacuum pump and by means of a three-way valve, either with thevacuum pump or with the second valve bounding the entry to the chamberof constant volume.
 4. Apparatus as claimed in claim 3, wherein all theducts forming part of the sample carrying chamber are capillary tubes.5. Apparatus as claimed in claim 4 including a calibrated spaceconnected by means of a valve to the chamber of constant volume so as todetermine the volume of the two chambers and the capillary tubes. 6.Apparatus as claimed in claim 5, including two clocks, interconnectedfor controlling the opening and closing of said first and said secondvalve.
 7. Apparatus as claimed in claim 6, wherein the calibrated space,the manostat and the sample carrying chamber are made of transparentmaterial.